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Chapter 9
Calculating Drug Dosages
Copyright © 2013, 2010, 2006, 2003, 2000, 1995, 1991 by Mosby, an imprint of Elsevier Inc.
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Learning Objectives
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Use formulas to determine the dosages of
tablets, capsules, or liquids
Use formulas to determine the total number
of tablets or capsules or the amount of liquid
to be ordered for a specified time
Use information about the apothecaries',
metric, and household measurements
systems to accurately calculate drug dosages
Copyright © 2013, 2010, 2006, 2003, 2000, 1995, 1991 by Mosby, an imprint of Elsevier Inc.
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Learning Objectives (cont.)
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
Calculate dosages for parenteral injections,
including those for special preparations such
as insulin
Calculate flow rates for infusions
Copyright © 2013, 2010, 2006, 2003, 2000, 1995, 1991 by Mosby, an imprint of Elsevier Inc.
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Calculating Medication Dosages
Three Steps
1. Verify that the drug available is the same
measurement system as the drug
dosage desired (convert if needed)
2. Reduce to lowest terms
3. Calculate the dosage quantity to be
administered
Copyright © 2013, 2010, 2006, 2003, 2000, 1995, 1991 by Mosby, an imprint of Elsevier Inc.
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Drug Calculation Methods
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Fraction Method
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600 mg = 200 mg
x tablets
1 tablet
Solve for x
Ratios or Proportion Method
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600 mg : x tablets :: 200 mg : 1 tablet
Solve for x
Copyright © 2013, 2010, 2006, 2003, 2000, 1995, 1991 by Mosby, an imprint of Elsevier Inc.
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Drug Calculation Methods (cont.)
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Desired over Available Method
Desired units (conversion factor) ×
Quantity of drug form = Quantity to give
Quantity available (× conversion factor)
Copyright © 2013, 2010, 2006, 2003, 2000, 1995, 1991 by Mosby, an imprint of Elsevier Inc.
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Forms of Oral Medications
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Capsules
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Cannot be broken or divided
If amount to be given is more than 0.5, round to next
whole number
Tablets
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Only divide if scored
 Coated tablets are not to be broken
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Liquids
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May be measured in a medication cup, syringe, or
calibrated dropper
Copyright © 2013, 2010, 2006, 2003, 2000, 1995, 1991 by Mosby, an imprint of Elsevier Inc.
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Scored and Unscored Tablets
Copyright © 2013, 2010, 2006, 2003, 2000, 1995, 1991 by Mosby, an imprint of Elsevier Inc.
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Parenteral Medications
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Medication available in three forms:
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Prefilled syringe labeled with specific dosage
• For example: meperidine (Demerol) 100 mg in 1 mL
Single-dose ampule or multiple-dose vial labeled
with a specific dosage per volume
• For example: epinephrine (Adrenalin) 1:1000 in 0.1 mL
A vial with powder that requires a specific fluid be
added to it to obtain a specific dosage
(Reconstitution)
Copyright © 2013, 2010, 2006, 2003, 2000, 1995, 1991 by Mosby, an imprint of Elsevier Inc.
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Insulin
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A critical medication that replaces the insulin
not being produced by the patient’s pancreas
Insulin comes in a standardized measure
called a “Unit”
Smallest amounts may be given; errors are
critical
Copyright © 2013, 2010, 2006, 2003, 2000, 1995, 1991 by Mosby, an imprint of Elsevier Inc.
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U-100 Vial
Copyright © 2013, 2010, 2006, 2003, 2000, 1995, 1991 by Mosby, an imprint of Elsevier Inc.
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Insulin (cont.)
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Strengths
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U-100 (100 Units of insulin per 1 mL)
U-500 (500 Units of insulin per 1 mL)
• Preparation 5 times stronger, rarely used
Syringe
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Calibrated in Units also
Tuberculin syringe used in emergency
• Minims used; 16 minims = 1 mL
Copyright © 2013, 2010, 2006, 2003, 2000, 1995, 1991 by Mosby, an imprint of Elsevier Inc.
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U-100 Syringe
Copyright © 2013, 2010, 2006, 2003, 2000, 1995, 1991 by Mosby, an imprint of Elsevier Inc.
13
Tuberculin Syringe
Copyright © 2013, 2010, 2006, 2003, 2000, 1995, 1991 by Mosby, an imprint of Elsevier Inc.
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Intravenous Medications
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Medications administered into the vein
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IV push
IV hanging by gravity (flow rate formula)
IV pump (mL/min or hr)
Copyright © 2013, 2010, 2006, 2003, 2000, 1995, 1991 by Mosby, an imprint of Elsevier Inc.
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Flow Rate Formula
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Gtts/min =
Volume to be administered × gtt factor
Time in minutes
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Drop factor of tubing:
Macrodrip = 10, 15, or 20 gtt/mL
Microdrip = 60 gtt/mL
Copyright © 2013, 2010, 2006, 2003, 2000, 1995, 1991 by Mosby, an imprint of Elsevier Inc.
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Question 1
The patient is prescribed prednisone 60 mg
PO once daily. The pharmacist sends 20 mg
tablets to the nursing unit. How many tablets
should the LPN give?
1.
2.
3.
4.
1 tablet
2 tablets
3 tablets
4 tablets
Copyright © 2013, 2010, 2006, 2003, 2000, 1995, 1991 by Mosby, an imprint of Elsevier Inc.
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Question 2
Ancef 750 mg is ordered. The vial is labeled
500 mg/1mL. How many mL are needed?
1.
2.
3.
4.
0.5 mL
1 mL
1.5 mL
2 mL
Copyright © 2013, 2010, 2006, 2003, 2000, 1995, 1991 by Mosby, an imprint of Elsevier Inc.
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Question 3
When mixing regular and NPH insulin:
1.
2.
3.
4.
Draw up the NPH insulin first.
Draw up the regular insulin first.
Inject air into the regular insulin first.
These two insulins should not be mixed.
Copyright © 2013, 2010, 2006, 2003, 2000, 1995, 1991 by Mosby, an imprint of Elsevier Inc.
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Learning Objectives
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List the rule used to calculate medication
dosages for children
Calculate flow rates for infusions for children
Copyright © 2013, 2010, 2006, 2003, 2000, 1995, 1991 by Mosby, an imprint of Elsevier Inc.
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Clark’s Rule
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Formula
Weight of the child × Adult dose = Child’s dose
Weight of the adult
Copyright © 2013, 2010, 2006, 2003, 2000, 1995, 1991 by Mosby, an imprint of Elsevier Inc.
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Body Surface Area
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Body surface area (BSA) = the total tissue area
A nomogram is used to easily calculate the BSA
in square meters
BSA formula
Surface area of the child (M2) × Usual adult dose
Surface area of an adult (1.73 M2) = Child’s dose
Copyright © 2013, 2010, 2006, 2003, 2000, 1995, 1991 by Mosby, an imprint of Elsevier Inc.
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Dimensional Analysis
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Steps
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Numbers in the dosage calculation problem are
placed on a grid along with their labels
The labels are cross-canceled to assure only one
label is left (one for answer)
Numbers in calculation are placed along grid next
to their labels
Copyright © 2013, 2010, 2006, 2003, 2000, 1995, 1991 by Mosby, an imprint of Elsevier Inc.
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Dimensional Analysis (cont.)
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Numbers are cross-canceled
Numbers are multiplied across the top and bottom
of the grid to yield a fraction
The fraction is divided, and the remaining label is
applied to the answer
Copyright © 2013, 2010, 2006, 2003, 2000, 1995, 1991 by Mosby, an imprint of Elsevier Inc.
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